#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
const ll LOG = 31;
const ll MOD = 1000000007;
const ll inf = 1e17;
const ll B = 2305843009213693951;
#define db(x) cerr << #x << " = " << x << " | "
#define dbg(x) cerr << #x << " = " << x << "\n"
#define Algerian ios::sync_with_stdio(0);
#define OI cin.tie(NULL);
int main() {
Algerian OI
ll n;
cin >> n;
vector<ll> x(n), y(n);
for (ll& c : x) cin >> c;
for (ll& c : y) cin >> c;
// deque<ll> dq;
set<ll> s;
for (ll i = 0; i < n; i++) s.insert(i);
ll res = 0;
for (ll i = n - 1; i >= 0; i--) {
while (y[i]) {
if (s.upper_bound(i) == s.begin()) {
ll c = *(--s.end());
ll take = min(x[c], y[i]);
x[c] -= take;
y[i] -= take;
res -= take;
if (!x[c]) s.erase(c);
} else {
auto it = --s.upper_bound(i);
if (*it == i && it != s.begin()) --it;
ll c = *it;
ll take = min(x[c], y[i]);
y[i] -= take;
x[c] -= take;
res += take * (c != i);
if (!x[c]) s.erase(c);
}
}
}
cout << res << "\n";
return 0;
}
